Foucault knife-edge test

The Foucault knife-edge test is a test of optical surfaces, developed by Léon Foucault,
that uses reflection and geometrical optical principles to amplify shadows
of defects on a telescope mirror so that they are made easily visible. Like
John Hadley some 200 years earlier, Foucault
placed a pinhole source at the mirror's center of curvature and arranged
the image to be formed alongside the source. However, unlike Hadley, Foucault
examined the rays converging to a focus by placing his eye behind a knife-edge,
which he then slowly introduced into the image. If the surface of the mirror
darkened uniformly, he knew the mirror was spherical; if it didn't, he was
able to deduce where and by how much the mirror surface deviated from sphericity.
This technique, now known as the Foucault knife-test, is incredibly sensitive:
bulges or hollows in a mirror surface with a relief as little as one millionth
of an inch are easily detectable. Armed with his knife-edge, Foucault was
able to produce mirrors with an accuracy of figure never before achieved.
His technique is still much used today.